Edge Termination by Ion Implantation in GaN

ABSTRACT

An edge terminated semiconductor device is described including a GaN substrate; a doped GaN epitaxial layer grown on the GaN substrate including an ion-implanted insulation region, wherein the ion-implanted region has a resistivity that is at least 90% of maximum resistivity and a conductive layer, such as a Schottky metal layer, disposed over the GaN epitaxial layer, wherein the conductive layer overlaps a portion of the ion-implanted region. A Schottky diode is prepared using the Schottky contact structure.

INCORPORATION BY REFERENCE

All patents, patent applications and publications cited herein arehereby incorporated by reference in their entirety in order to morefully describe the state of the art as known to those skilled therein asof the date of the invention described herein.

FIELD OF THE INVENTION

The present invention relates to edge termination in devices. Inparticular, the present invention relates to edge termination of devicesexperiencing high electric field at their otherwise untreated edges.More particularly, the present invention relates to edge terminationsuitable for use in GaN devices.

BACKGROUND OF THE INVENTION

In order to achieve the maximum possible voltage handling capability ofa metal-semiconductor Schottky barrier contact, the contact shouldapproach its behavior as if it were an infinite parallel plane. Such atheoretical structure would have a uniform electric field at itsinterface, increasing with applied voltage until it reaches the criticalelectric field for the onset of impact ionization in the underlyingsemiconductor. Since it is not possible to have an infinite extent inpractice, and finite edges will exist with increased electric field dueto curvature of equipotential lines at the contact corner leading tofield crowding, the edge of the terminal must be treated in such a wayas to make it behave as close to the ideal case as possible. Onepossible way to accomplish this is by reducing the potentialdistribution gradients and thus electric field. Another possibility isto alleviate the effect of the increase in electric field.

Various techniques such as doped guard rings, field plates, and mesaetching have been used for edge termination. Doped guard rings aredifficult to form in GaN due to the high temperature activation annealfor implanted dopants, and the low diffusion rates of dopant species.Field plates improve the edge termination but do not approach the idealparallel plane breakdown voltage. Mesa etching removes desirableplanarity, and requires careful surface passivation to maintainstability of its effect. Mesa edge termination also falls short of theideal breakdown voltage.

SUMMARY OF THE INVENTION

An edge termination technique is described which can be used to achieveimproved breakdown voltage for GaN devices. This technique involvesimplanting a neutral species on the edges of devices to form ahigh-resistive layer. In accordance with the present invention, Schottkycontacts and Schottky diodes having increased breakdown voltage aredescribed.

In one or more embodiments, an insulating layer is created by ionimplantation of a non-electrically active species to create latticedamage. Isoelectronic N implantation, for example, can be used to createan electrically isolating region encompassing the location of peakelectric field. Such an isolating region will alleviate the onset ofavalanche breakdown due to impact ionization.

In one embodiment, the implantation profile may consist of multipleimplant energies and doses designed to result in a uniform concentrationof the implant species to the desired depth. Such a profile will resultin damage to the lattice throughout the implanted layer.

The dose is designed to create sufficient damage to result in highresistivity. One example is to implant isoelectronic N with 10-600 keVenergies, with doses in the range of 10¹² to 10¹⁴ cm⁻². Excessive damagecan result in trap level assisted conduction and hence a less isolatingregion. A thermal anneal may be performed to the control the tail of thedistribution, or for subsequent device processing steps, but not to theextent of repairing the desirable lattice damage for isolation purposes.

In one or more embodiments, the effectiveness of the implant edgetermination can be increased by combination with a field plate. Such afield plate will serve to further decrease the electric field at thecontact metal corner by spreading the field across an additionaldeposited dielectric.

In one aspect, an edge terminated semiconductor device includes a GaNsubstrate; a doped GaN epitaxial layer deposited on the GaN substratecomprising an ion-implanted insulation region, wherein the ion-implantedregion has a resistivity that is at least 90% of maximum achieveableresistivity and a conductive layer disposed over the GaN epitaxiallayer, wherein the conductive layer overlaps a portion of theion-implanted region.

In another aspect, an edge terminated semiconductor device includes aGaN substrate; a doped GaN epitaxial layer grown on the GaN substratewhich includes an ion-implanted insulation region. The ion(s) used tocreate the ion-implanted region are selected from the group consistingof helium, argon, nitrogen or other elements and/or combinationsthereof. A conductive layer is disposed over the GaN epitaxial layer,wherein the conductive layer overlaps a portion of the ion-implantedregion.

In another aspect, an edge terminated semiconductor device, includes aGaN substrate; a doped GaN epitaxial layer deposited on the GaNsubstrate comprising an ion-implanted insulation region, wherein theions are implanted to a depth of at least 400 nm, and a conductive layerdisposed over the GaN epitaxial layer, wherein the conductive layeroverlaps a portion of the ion-implanted region.

In one or more embodiments, the ion-implanted region has a depth of atleast 400 nm.

In one or more embodiments, the ion-implanted region has a resistivitygreater than 10¹⁰ ohm-sq.

In one or more embodiments, the GaN epitaxy layer is n-doped.

In one or more embodiments, the mean ion-implanted ion concentration is>10¹⁸ cm⁻³.

In one or more embodiments, the ion-implanted region comprises adistribution of implanted ions.

In one or more embodiments, the conductive layer comprises a Schottkymetal, and for example, the Schottky metal is selected from the groupconsisting of nickel, platinum, palladium and gold and combinationsthereof.

In one or more embodiments, the conductive layer comprises p+ GaN.

In one or more embodiments, the overlap of the conductive layer over theion-implanted insulation region is about 0.5 μm to about 10 μm.

In one or more embodiments, the implant ion comprises nitrogen.

In one or more embodiments, the device further includes an insulatinglayer disposed over the ion-implanted insulation region, wherein theconductive layer overlaps both a portion of the ion-implanted insulationregion and a portion of the insulating layer.

In one or more embodiments, the device is a diode, and for example thediode is selected from the group of Schottky, PiN, PN, and mergedPiN-Schottky diodes.

In one or more embodiments, the device is a field effect transistor andfor example, the field effect transistor is a vertical junction fieldeffect transistor.

In another aspect, a method of making an edge terminated semiconductordevice includes providing a GaN substrate having a GaN epitaxial layergrown thereon; exposing a portion of the GaN epitaxial layer to ionimplantation, wherein the energy dose is selected to provide aresistivity that is at least 90% of maximum achievable resistivity; anddepositing a conductive layer over a portion of the implanted region.

In one or more embodiments, exposing the GaN epitaxy layer to ionimplantation includes multiple implants at different energies and/ordosages selected to create a uniform layer of lattice damage.

In one or more embodiments, the multiple implants includes 2 or moreimplants at increasing energy and/or different doses.

In one or more embodiments, exposing the GaN epitaxy layer to ionimplantation includes exposing the layer to one or more implantsselected to provide an implant depth of greater than 450 nm.

In one or more embodiments, the implant ion is selected from the groupconsisting of nitrogen, argon, helium, other elements, and mixturesthereof.

In one or more embodiments, the conductive layer comprises a Schottkymetal.

In one or more embodiments, the conductive layer comprises p+ GaN.

In one or more embodiments, the method further includes prior todepositing the conductive layer, depositing an insulating layer over theion-implanted insulation region, wherein upon subsequent deposition ofthe conductive layer, the conductive layer overlaps both a portion ofthe ion-implanted insulation region and at least a portion of theinsulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe apparent upon consideration of the following detailed description,taken in conjunction with the accompanying drawings, in which likereference characters refer to like parts throughout, and in which:

FIG. 1 is a cross sectional illustration of a GaN n+ substrate,including n− GaN epitaxial layer, for use in preparing the Schottkycontact according to one or more embodiments described herein.

FIG. 2 is a cross sectional illustration of a GaN n+ substrate,including n− GaN epitaxial layer, with a blanket Ohmic contact formed onthe substrate.

FIG. 3 is a cross sectional illustration of a GaN n+ substrate,including n− GaN epitaxial layer, illustrating formation of an ionimplanted region according to one or more embodiments described herein.

FIG. 4 is a cross-sectional view of a Schottky contact and insulatingimplant area according to one or more alternative embodiments.

FIG. 5 is a cross sectional illustration of a GaN n+ substrate,including n− GaN epitaxial layer and ion implanted region, onto which aninsulating layer is deposited, according to one or more embodimentsdescribed herein.

FIG. 6 is a cross section of GaN-based vertical Schottky diode withcombined field plate and ion implanted edge termination in accordancewith certain embodiments of the present invention; and

FIG. 7 is a cross sectional illustration demonstrating the use of ametal mask in the preparation of a Schottky contact or diode accordingto one or more embodiments.

FIG. 8 is a cross-sectional view of a PIN diode according to one or moreembodiments of the invention.

FIG. 9 is a cross-sectional view of an implant edge termination forvertical junction field effect transistor (FET) according to one or moreembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An electric field is set up between positively and negatively chargedregions in a device. Under ideal conditions, the electric field at aplanar junction will be uniform everywhere as if it is contained withinan infinite parallel plane. Since it is not possible to have an infiniteextent in practice, finite edges will exist with increased electricfield due to crowding of equipotential lines at the contact corner.

Edge termination for devices having an elevated field region isdescribed. Edge termination is accomplished using ion implanted neutralspecies on the edges of the device to form a resistive layer. A lightlydoped region under the edges of the layer creating the electric field isused to shape the potential distribution, since the potential gradientis proportional to the doping level. The lower gradient of potentiallowers the peak electric field at the junction. Placing lightly doped orinsulating material under the edge of the layer creating the electricfield also increases the critical field for onset of impact ionizationcompared to the semiconductor material that would otherwise exist inthat location. Such an insulating layer can extend sufficiently farunder and beyond the corner to encompass the entire elevated fieldregion.

This technique is applicable to any device including regions of elevatedelectric field, which typically occur at the edge of devices. Forexample, it can be incorporated into devices having p-n junction layersor metal-semiconductor junction layers. In one or more embodiments, itcan be incorporated into devices such as diodes, e.g., Schottky diodes,p-n diodes, pin diodes, etc. and transistors, e.g., FET, JFET, MOSFETetc. that include corners or localized potential gradients leading tohigher electric field. In particular, the technique is particularlysuitable for overcoming the processing and technological limitations ofGaN devices.

FIG. 4 is a schematic illustration of GaN device including an implantedisolation region 301 according to one or more embodiments describedherein. The implant depth is chosen to shape the potential distributionof the electric field under the conductive layer 401 and lower themaximum electric field as much as possible. The conductive layer can bea metal, such as used for example in a Schottky diode, or it can be adoped semiconductor layer, such as used for example in transistors andp-n diodes. The placement of conductive layer 401 over a portion 402 ofthe implanted region can be selected to lower the electric field at theplate edge or to contain the entire elevated field region under themaximum bias allowed and thereby increase the critical field for onsetof impact ionization in the high field region.

One method of creating an insulating region is to use ion implantationof a non-electrically active species to create lattice damage. A damagedcrystal lattice will contain allowed energy levels, or traps, within theforbidden energy gap of the perfect crystal. A large number of deeptraps will make the region highly resistive. Excessive damage, however,can result in trap to trap conduction and reduce the resistance of thedamaged region. A wide range of ions could be used to achieve thisdamage, such as nitrogen (N), helium (He), iron (Fe) or Argon (Ar). Apreferred ion species for GaN is nitrogen (N) since it is isoelectronic,or already present in the target crystal. Isoelectronic N implantationinto the GaN device layer can be used to create an electricallyisolating region encompassing the location of peak electric field underthe corner of the conductive contact as shown in FIG. 4.

To fabricate the final device in FIG. 4, one starts with an n+ GaNsubstrate (100) with a typical doping level of 10¹⁷-10²⁰ cm⁻³ and alightly doped n− epitaxial layer (101) as depicted in FIG. 1. Theepitaxial layer doping may range from 10¹⁴ to 10¹⁷ cm⁻³, for example,and is typically n-type due to higher electron mobility for electrons inGaN. The dopant is typically Si for intentional doping, and well-knowntechniques for reducing the as-grown background components in GaN (whichtypically render it n-type with low net concentration) can also be usedin preparing the epilayer. The thickness of the epitaxial layer isnormally in the range of 1 μm to 100 μm, depending on the device design.The desired doping concentration and thickness is formed by epitaxialgrowth on a GaN substrate according to well-known techniques.

FIG. 3 shows the location of ion implantation region (301) intended foredge termination of the final device. Implantation has an approximatelyGaussian roll-off from the mask edge used to define its location, whichis symbolized by the bevel edge in FIG. 3. The location of the implantis typically defined using a mask defined by photolithography. The maskmaterial is capable of blocking the implanted species in areas where itis not desired.

Photoresist is patterned on the surface to define regions for ionimplantation. The photoresist is designed to be thick enough to blockthe maximum energy implant from reaching the semiconductor surface. Theimplant is not blocked in the areas where photoresist has been removed.The purpose of the implantation is to form an electrically isolatinglayer.

Alternatively, the implant mask can be formed by patterning metaldeposited on surface instead of photoresist. One example is to usenickel in place of photoresist for implantation mask. In someembodiments, the metal mask is a Schottky metal. FIG. 7 illustrates aSchottky metal layer (701) that has been deposited over the epitaxialGaN layer. In case of using the Schottky metal for an implant mask, themetal is not removed after implant. Implantation results in lateralspreading of the implant region underneath the edge of the overlayingconductive layer, so that the desired overlap of overlaying conductivelayer and underlying implant isolation region is obtained.

The implanted species could consist of a wide range of ionized elements,but lightweight inert ions such as nitrogen or helium are preferred forgreater penetration depth with implant energy. For example, at 400 keVthe mean implantation depth for Argon is 0.2 μm in GaN, whereas the meanimplant depth at the same energy is 0.5 μm for nitrogen and 1.1 μm forhelium. The implanted layer could comprise multiple implants atdifferent energies and doses designed to create a uniform layer oflattice damage. The dose should be designed to create sufficient damageto result in high resistivity, but not so high as to create excessivedamage. The dosage regimen desirably produces an implanted region havingat least 90% of maximum achieveable resistivity. Maximum resistivity isreadily determined for a given material by measurement over a range ofimplant energy doses. The maximum resistivity can vary for differentmaterials and implanted elements, but typically is greater than 10¹⁰ohm/sq. Exposure to dosage in excess of that which is sufficient toachieve maximum resistivity, leads to the creation of excessive deeptraps and hopping conduction. The presence of additional damageindicative of exposure beyond that needed to achieve maximum resistivityis demonstrated by a high leakage current during operation of thedevice. Excessive lattice damage can also be inferred by observationsusing transmission electron microscopy (TEM), using Rutherfordbackscattering or SIMS measurements, for example.

In order to obtain an ion implanted isolation region having uniformlattice damage, the region can be treated with multiple ion implants atdifferent doses. The summation of the Gaussian-like distributions can bedesigned to result in a nearly uniform concentration from near thesurface to the maximum implant depth. One example of an implantationrecipe for helium intended to create a uniform concentration 1.5 μmthick isolation layer include four implant energies of 20 keV, 60 keV,160 keV, 400 keV with 6.8×10¹³ cm⁻², 1.8×10¹⁴ cm⁻², 3.1×10¹⁴ cm⁻², and4.3×10¹⁴ cm⁻² dose, respectively.

A thermal anneal may be performed to control the tail of thedistribution. Thermal anneal can repair lattice damage and can be usedto refine the location of the implanted region. By partial annealing,the lightly damaged tail of the Gaussian distribution of damage can berepaired while the heavily damaged region is relatively untouched. Thethermal anneal should not repair the implant region to the extent ofrepairing the desirable lattice damage introduced for isolationpurposes. Such an isolating region will alleviate the onset of avalanchebreakdown due to impact ionization, by lowering the maximum electricfield present, and by increasing the critical field for breakdown.

Referring now to FIG. 4, the final processing step consists ofdeposition and patterning of the conductive layer. The placement ofconductive layer 401 over a portion of the implanted region is designedto contain the entire elevated field region under the maximum biasallowed on the device.

In some embodiments, the conductive layer can be a metal and the deviceis a Schottky diode. The Schottky metal (401) is typically Nickel,Platinum, Palladium, or Gold, which are metals known to form arelatively high (>0.9 eV) Schottky barrier on GaN. The metal ispatterned to overlap the implanted isolation region to an extent whichwill encompass the elevated electric field region at the corner of themetal. The metal overhang is a function of the anticipated electricfield flux (excursion) and is typically in the range of 0.5-10 μm. Theanticipated field excursion can be determined, for example, usingsimulation or models and the material overhang can be selected toenclose the predicted field excursion. The Schottky metal layerthickness is typically >0.2 μm, or greater than 0.5 μm, but the metalstack may be as thick as desired.

In other embodiments, the conductive layer can be a GaN p+ layer, andthe junction is a p-n junction. Devices including p-n junctions includePIN diodes, merged P-i-N/Schottky diodes, transistor structures such asMOSFET, JFET and IGBT and other devices such as thyristors as will beevident to those skilled in the art.

A backside Ohmic contact (201) is also deposited and annealed. Thecontact is generally Ti/Al based and annealed at >700° C. for >5 min,but other possibilities exist. See, FIG. 2.

The edge termination method is a relatively simple processing stepinvolving only an implant mask, the ion implantation itself, and maskremoval. This method also maintains the planarity of the device.Non-planarity introduces complexity for subsequent processing steps.This method is effective as part of a solution which can increasebreakdown voltage to a value approaching that of an infinite parallelplane diode, unlike many of the alternatives. This technique also hasthe advantage of being compatible with GaN, while many standard siliconprocess techniques such as selective area doping, oxidation, and wetetching are not readily available.

In another aspect, a vertical GaN-on-GaN Schottky diode with anion-implanted edge termination region is described. A Schottky barrieris a potential barrier formed at a metal-semiconductor junction whichhas rectifying characteristics. Schottky barriers are incorporated intoSchottky diodes and transistors, as well as other devices. When themetal and n-type semiconductor contact one another, some of theelectrons in the semiconductor (which are at a higher energy) movespontaneously into the metal and collect on the surface. These leavebehind ionized donor locations, which are positively charged, and createa space charge region in the semiconductor. An electric field is set upbetween these positive charges and the electrons and a barrier iscreated that eventually inhibits further electron flow into the metal.This barrier and electric field are increased when negative bias isapplied to the metal. The maximum reverse bias voltage that can beapplied to a diode is limited by breakdown. Breakdown is characterizedby the rapid increase of the current under reverse bias. Thecorresponding applied voltage is referred to as the breakdown voltage.The breakdown voltage is an important parameter of power devices;however, it is also relevant to logic and other devices.

A GaN Schottky diode will outperform competitors in Si and SiC due tothe outstanding material properties of GaN. Homoepitaxial GaN layers onbulk GaN substrates are imbued with superior properties to othermaterials being used and considered for power electronic devices. Highelectron mobility, μ, is predicted for a given background doping level,N, which implies low resistivity, ρ, since ρ=1/qμN. Another outstandingproperty is that of high critical electric field, E_(crit), foravalanche breakdown. A high critical electric field allows largevoltages to be supported over smaller length, L, than a material withlesser E_(crit). A shorter distance for current to flow and a lowresistivity give rise to a lower resistance, R, than high voltagedevices in a less favorable material since R=ρL/A. In general, there isa tradeoff between the physical dimension needed to support high voltagein the device's off-state, and the ability to pass current through thesame region with low resistance in the on-state. Several figures ofmerit have been developed to represent this tradeoff, with GaN shown tobeat all viable competitors.

GaN layers grown on bulk GaN substrates have low defect density comparedto layers grown on mismatched substrates. The low defect density willgive rise to superior thermal conductivity, and is also expected toresult in less trap related effects such as dynamic on-resistance. Oneway to take advantage of these properties is with a Schottky barrierdiode.

The Schottky diode is formed by an interface between lightly n-typedoped GaN and a metal with a larger work function than the GaN such asNickel, Palladium, Platinum or Gold. Lightly doped n-type GaN is chosenfor its superior electron mobility compared to that of holes in a p-typematerial. The work function difference forms a rectifying barrier whichcan surmounted by applying forward bias, but blocks current in reversebias up to a maximum voltage determined by the properties of the GaNepitaxial layer, provided edge termination is provided. The forwardcurrent is due to majority carrier flow, so there are no minoritycarrier storage effects as there are in a p-n junction.

For power devices, a vertical structure often has advantages overlateral one in improving current density and breakdown voltage. Avertical current flow GaN Schottky diode has favorable trade-offsbetween resistance, thermal conductivity and high voltage operation. Inorder to fully utilize high critical electrical field of GaN material,special care is needed to design the Schottky metal contact, especiallyat the metal corner on n-type GaN material to avoid an excessiveelectrical field during high voltage operation.

A vertical GaN-on-GaN Schottky diode with combined ion-implanted edgetermination region and field plate is shown in FIG. 6. By combining anion-implanted edge termination layer (301) and insulator layer (501),the corner electrical field between Schottky metal contact layer (601)and n-type GaN epitaxial layer (101) can be further alleviated comparedto prior art devices. The implant depth is chosen to shape the potentialdistribution under the Schottky contact and lower the maximum electricfield as much as possible. The placement of Schottky metal 601 over aportion of the implanted region 301 is designed to contain the entireelevated field region under the maximum bias allowed on the Schottkycontact.

The effectiveness of the implant edge termination can be increased bycombination with a field plate formed by overlapping a Schottky metal601 over a deposited insulator 501, as shown in FIG. 6. The additionalfeatures in this embodiment over that of FIG. 4 are the field plateoverhang and the field plate height. The field plate height isdetermined by the thickness of insulating layer 501. Such a field platewill serve to further decrease the electric field at the contact metalcorner by spreading the field across an additional deposited dielectric.The appropriate field plate overhang and plate thickness are determinedby simulation or experiment to minimize the electric field at theSchottky metal corner by shaping the potential distribution andspreading the electric field. In addition the insulating material cantypically withstand a higher electric field than the semiconductorwithout breaking down.

To fabricate the final device in FIG. 6, one starts with an n+ GaNsubstrate with a typical doping level of 10¹⁷-10²⁰ cm⁻³, and an n−epitaxial layer, as depicted in FIG. 1. In other embodiments, p-dopedGaN can be used for the epitaxial layer. The epitaxial layer doping mayrange from 10¹⁴ to 10¹⁸ cm⁻³, for example, and is typically n-type dueto higher electron mobility for electrons in GaN. The thickness of theepitaxial layer is normally in the range of 1 μm to 100 μm, depending onthe device design. A backside ohmic contact (201) is also deposited andannealed. The contact is generally Ti/Al based and annealed at >700° C.for >5 min. Further details are presented above in discussion of FIG. 4.

The epitaxial layer is treated to form the high resistivity regionintended for edge termination of the final device as describedpreviously. The location of the implant is defined by using a maskdefined by photolithography. The mask material is capable of blockingthe implanted species in areas where it is not desired. The implantedspecies could consist of a wide range of ionized elements, but lessmassive, inert ions such as nitrogen and helium are preferred. Theimplanted layer could be composed of multiple implants at differentenergies, typically 10-600 keV, and doses, typically 10¹² to 10¹⁵ cm⁻²,designed to create a uniform layer of lattice damage. One example of animplantation recipe for helium comprised of four implant energies tocreate a uniform concentration 1.5 μm thick isolation layer is 20 keV,60 keV, 160 keV, 400 keV with 6.8×10¹³ cm⁻², 1.8×10¹⁴ cm⁻², 3.1×10¹⁴cm⁻², and 4.3×10¹⁴ cm⁻² dose, respectively. The dose should be designedto create sufficient damage to result in high resistivity, but not sohigh as to create excessive damage. A thermal anneal may be performed tothe control the tail of the distribution, or in subsequent processingsteps, but not to the extent of repairing the desirable lattice damagefor isolation purposes. Such an isolating region will alleviate theonset of avalanche breakdown due to impact ionization, by lowering themaximum electric field present, and by increasing the critical field forbreakdown. Further details are presented above in discussion of FIG. 3.

The next processing step involves deposition and patterning of aninsulator (501), as illustrated in FIG. 5. The insulator material may beSiN_(x) or SiO_(x), or a number of other acceptable insulatingmaterials. The insulator thickness is chosen to determine the fieldplate height created by later Schottky metal deposition. The insulatoris patterned to define the edge of the Schottky barrier and is locatedsufficiently far inside the implanted region to contain the highelectric field at the Schottky metal corner. Typically the insulator isset back about 1-5 μm from the edge of the implanted region.

The final processing step consists of deposition and patterning of theSchottky metal (601), as shown in FIG. 6. The placement of Schottkymetal 601 over a portion of the implanted region is designed to containthe entire elevated field region when subjected to the maximum biasallowed on the Schottky contact. Typically the Schottky metal is Nickel,Platinum, Palladium, or Gold, which are metals known to form arelatively high (>0.9 eV) Schottky barrier on GaN. The metal ispatterned to overlap the deposited insulator layer to an extent which isdesigned to minimize electric field present at the corner of the metalthat is contacting the implanted semiconductor region. The combinationof the alleviation of the electric field by the field plate and theproperties of the implanted layer described earlier are expected tocombine and allow near parallel plane breakdown properties for the finaldevice.

Diodes can also be formed using GaN with opposite polarities, e.g.p-type and n-type, in which the electric field is generated at theinterface between highly doped (e.g., dopant concentrations of greaterthan 10¹⁷ cm⁻³) and a lighter doped layer of the opposite doping type.The electric fields generated at the interface can be alleviated by edgetermination. In one or more embodiments, an edge termination is providedfor a PIN diode in the region below the p+ GaN edge. With reference toFIG. 8, a PIN diode according to one or more embodiments is described.The PIN diode include an n+ GaN substrate (800) with a typical dopinglevel of 10¹⁷-10²⁰ cm⁻³ and a lightly doped (intrinsic) n− epitaxiallayer (801). The epitaxial layer doping may range from 10¹⁴ to 10¹⁷cm⁻³, for example, and is typically n-type due to higher electronmobility for electrons in GaN. The dopant is typically Si forintentional doping, and well-known techniques for reducing the as-grownbackground components in GaN (which typically render it n-type with lownet concentration) can also be used in preparing the epilayer. Thethickness of the epitaxial layer is normally in the range of 1 μm to 100μm, depending on the device design. The desired doping concentration andthickness is formed by epitaxial growth on a GaN substrate according towell-known techniques.

A p+ GaN layer (802) with a typical doping level of 10¹⁷-10²⁰ cm⁻³overlays the intrinsic layer and can be patterned using conventionalmasking and etching techniques, or formed by selective regrowth upperand lower Ohmic contact metal layers (803, 804) are deposited to provideelectrical contact to the device. According to one or more embodiments,an ion implanted region (810) is located below the edge of theconductive p+ GaN layer. The implanted species could consist of a widerange of ionized elements, but lightweight inert ions such as nitrogenor helium are preferred for greater penetration depth with implantenergy. For example, at 400 keV the mean implantation depth for Argon is0.2 μm in GaN, whereas the mean implant depth at the same energy is 0.5μm for nitrogen and 1.1 μm for helium. The implanted layer could becomprised of multiple implants at different energies and doses designedto create a uniform layer of lattice damage. The dose should be designedto create sufficient damage to result in high resistivity, but not sohigh as to create excessive damage. The dosage regimen desirablyproduces an implanted region having at least 90% of maximum achieveableresistivity. Maximum resistivity is readily determined for a givenmaterial by measurement over a range of implant energy doses. Themaximum resistivity can vary for different materials and implantedelements, but typically is greater than 10¹⁰ ohm/sq.

In order to obtain an ion implanted isolation region having uniformlattice damage, the region can be treated with multiple ion implants atdifferent doses. The summation of the Gaussian-like distributions can bedesigned to result in a nearly uniform concentration from near thesurface to the maximum implant depth. One example of an implantationrecipe for helium intended to create a uniform concentration 1.5 μmthick isolation layer includes four implant energies of 20 keV, 60 keV,160 keV, 400 keV with 6.8×10¹³ cm⁻², 1.8×10¹⁴ cm⁻², 3.1×10¹⁴ cm⁻², and4.3×10¹⁴ cm⁻² dose, respectively.

A thermal anneal may be performed to control the tail of thedistribution. Thermal anneal can repair lattice damage and can be usedto refine the location of the implanted region. By partial annealing,the lightly damage tail of the Gaussian distribution of damage can berepaired while the heavily damaged region is relatively untouched.

In other embodiments, the edge termination is incorporated into fieldeffect transistors having at least one p-n junction. The high electricfield or electric field spikes arising at the corner of the p-region arealleviated by introduction of a high resistivity region under the cornerof the p region.

FIG. 9 illustrates a vertical junction field effect transition (VJFET)that includes an n+ GaN substrate with a typical doping level of10¹⁷-10²⁰ cm⁻³ and a lightly doped (intrinsic) n− epitaxial layer withdoping ranging from 10¹⁴ to 10¹⁷ cm⁻³, for example. A p+ GaN layer witha typical doping level of 10¹⁷-10²⁰ cm⁻³ overlays the intrinsic layer toform gate and drain electrodes. An n-doped channel including a lightlydoped n− (intrinsic) layer and a highly doped n+ layer spans the gateand source electrodes. The FET controls the flow of electrons (or holes)from the source to drain by electrically modulating a conducting channelusing voltage (or lack of voltage) applied across the gate and sourceterminals. According to one or more embodiments, an ion implanted regionis located below the edge of the conductive p+ GaN layers making up thesource and drain electrodes. The implanted species could consist of awide range of ionized elements, but lightweight inert ions such asnitrogen or helium are preferred for greater penetration depth withimplant energy. For example, at 400 keV the mean implantation depth forArgon is 0.2 μm in GaN, whereas the mean implant depth at the sameenergy is 0.5 μm for nitrogen and 1.1 μm for helium. The implanted layercould be comprised of multiple implants at different energies and dosesdesigned to create a uniform layer of lattice damage. The dose should bedesigned to create sufficient damage to result in high resistivity, butnot so high as to create excessive damage. The dosage regimen desirablyproduces an implanted region having at least 90% of maximum resistivity.

Such devices can be prepared using conventional techniques as are knownin the art, and with consideration of the discussion above.

Upon review of the description and embodiments of the present invention,those skilled in the art will understand that modifications andequivalent substitutions may be performed in carrying out the inventionwithout departing from the essence of the invention. Thus, the inventionis not meant to be limiting by the embodiments described explicitlyabove, and is limited only by the claims which follow.

What is claimed is:
 1. An edge terminated semiconductor device,comprising; a GaN substrate; a doped GaN epitaxial layer grown on theGaN substrate including an ion-implanted insulation region, wherein theion-implanted region has a resistivity that is at least 90% of maximumachieveable resistivity and a conductive layer disposed over the GaNepitaxial layer, wherein the conductive layer overlaps a portion of theion-implanted region.
 2. An edge terminated semiconductor device,comprising; a GaN substrate; a doped GaN epitaxial layer deposited onthe GaN substrate including an ion-implanted insulation region, whereinthe ion of the ion-implanted region is selected from the groupconsisting of helium, argon, nitrogen, other elements and combinationsthereof, and a conductive layer disposed over the GaN epitaxial layer,wherein the conductive layer overlaps a portion of the ion-implantedregion.
 3. An edge terminated semiconductor device, comprising; a GaNsubstrate; a doped GaN epitaxial layer deposited on the GaN substrateincluding an ion-implanted insulation region, wherein the ions areimplanted to a depth of at least 450 nm, and a conductive layer disposedover the GaN epitaxial layer, wherein the conductive layer overlaps aportion of the ion-implanted region.
 4. The device of claim 1, whereinthe ion-implanted region has a depth of at least 450 nm.
 5. The deviceof claim 1 or 2, wherein the ion-implanted region has a resistivitygreater than 10¹⁰ ohm-sq.
 6. The device of claim 1, wherein the GaNepitaxy layer is n-doped.
 7. The device of claim 1, wherein the meanion-implanted ion concentration is >10¹⁸ cm⁻³.
 8. The device of claim 1,wherein the ion-implanted region comprises a distribution of implantedions.
 9. The device of claim 1, wherein the conductive layer comprises aSchottky metal.
 10. The device of claim 9, wherein the Schottky metal isselected from the group consisting of nickel, platinum, palladium andgold and combinations thereof.
 11. The device of claim 1, wherein theconductive layer comprises p+ GaN.
 12. The device of claim 1, whereinthe overlap of the conductive layer over the ion-implanted insulationregion is about 0.5 μm to about 10 μm.
 13. The device of claim 2,wherein the implant ion comprises nitrogen.
 14. The device of claim 1, 2or 3, further comprising: an insulating layer disposed over theion-implanted insulation region, wherein the conductive layer overlapsboth a portion of the ion-implanted insulation region and a portion ofthe insulating layer.
 15. The device of claim 11, wherein the device isa diode.
 16. The device of claim 15, wherein the diode is selected fromthe group of p-n diodes, PiN diodes, and merged PiN-Schottky diodes. 17.The device of claim 11, wherein the device is a field effect transistor.18. The device of claim 17, wherein the field effect transistor is avertical junction field effect transistor.
 19. A method of making anedge terminated semiconductor device, comprising: providing a GaNsubstrate having a GaN epitaxial layer grown thereon; exposing a portionof the GaN epitaxial layer to ion implantation, wherein the energy doseis selected to provide a resistivity that is at least 90% of maximumachieveable resistivity; and depositing a conductive layer over aportion of the implanted region.
 20. The method of claim 19, whereinexposing the GaN epitaxy layer to ion implantation comprises multipleimplants at different energies and/or dosages selected to create auniform layer of lattice damage.
 21. The method of claim 20, wherein themultiple implants comprises 2 or more implants at increasing energyand/or different doses.
 22. The method of claim 19, wherein exposing theGaN epitaxy layer to ion implantation comprises exposing the layer toone or more implants selected to provide an implant depth of greaterthan 450 nm.
 23. The method of claim 19, wherein the implant ion isselected from the group consisting of nitrogen, argon, helium, or otherelements, and mixtures thereof.
 24. The method of claim 19, wherein theconductive layer comprises a Schottky metal.
 25. The method of claim 19,wherein the conductive layer comprises p+ GaN.
 26. The method of claim19, further comprising: Prior to depositing the conductive layer,depositing an insulating layer over the ion-implanted insulation region,wherein upon subsequent deposition of the conductive layer, theconductive layer overlaps both a portion of the ion-implanted insulationregion and at least a portion of the insulating layer.